One of the companies I worked for made test instruments that were used during nuclear tests, starting in the 1950s, to calculate device yields. Some of the instruments and their sensors were very near the device.

A concrete wall protected the instruments from the blast wave for a short interval; during that interval, the signal had to make it from the sensor, through the instrument’s circuitry, and out the back-panel output connected to the cable to the test center, before the blast wave disintegrated the sensor, wall, instrument, and cable.

At the time, all we knew of the tests was that the government was buying a lot of our instruments, and that the instruments never returned for repair or calibration. One day, the test director visited our company, and requested that we alter the design for one of the instruments, in order to reduce the signal latency time (from front panel input to rear panel output) from 250 milliseconds to 8 msec.

One of the engineers said “Wow! Why do you need it that fast?”. The test director responded “Because that’s how long it takes 20 feet of concrete to evaporate.” Technically, concrete doesn’t evaporate; it undergoes spalling degradation at those pressures and temperatures. But we got his gist …

If you were 100 meters from the device, your head (which is less than 1/20th the thickness of a 20-foot-thick concrete wall) would completely disintegrate in 400 microseconds, less than the conduction delay of an axon in your brain. You would (literally) “never know what hit you”.

A nuclear explosion is different from other explosions, It’s really cool.

When the nuke inside the steel bomb casing is “activated,” the first thing it does is send out a radiation pulse that lights up the sky.

After that, the explosive overpressure and the heatwave spread out, cracking and disintegrating the steel casing of the bomb.

Being so close, perhaps that first pulse would probably overload your brain with electricity, so there is no useful electrical activity in your brain anymore.

So, you’re probably brain-dead before the bomb actually “explodes.” And then the force of the explosion would turn you into molecules and atoms, and the heat would eliminate any moisture, so a 150-pound person would become a flock of superheated carbon and hydrogen molecules, about 30 pounds in all, flying supersonic outwards.